This invention relates to low noise faulted current indicators for power lines, and particularly to faulted current indicators capable of rejecting the effects of fluxes emanating from adjacent power lines. The invention also pertains to improvements in the sensitivity of fault indicator sensors.
Faulted current indicators (or FCI's or fault indicators) are placed at intervals along power lines or cables to decrease the time that it takes line crews to locate faults in utility distribution systems. In general, each FCI is a high current trip and low current reset indicating device that displays whether a fault has occurred between the indicator and a load. Such devices operate by responding to fluxes generated by currents in a line.
Typically, a sensor includes a magnetic core with surrounding a coil winding that serves as a transformer secondary to read out the current in the cable which functions as a transformer primary. Efficient coupling requires a minimum series reluctance so that the flux swing is maximized at 50 or 60 Hz. Further, the power transfer also depends upon the "copper window" and axial coupling area of the core about the power cable.
A sensor's output is a function of N d.phi./dt and consequently the number of windings should be maximum for maximum signal voltage output. However, in three phase systems with but one sensor per indicator, the lines are not monitored in an "or" fashion, but rather, the individual lines indicate line fault and reset independent of the other phases. Thus, if a surge of current accompanying a short out fault in a given line trips an indicator and a fuse/circuit breaker then opens the line, an adjacent live phase with a sufficiently high current may falsely reset the open line indicator. Typically, the more sensitive the sensor is to the line to be sensed, the more sensitive that sensor is to external magnetic fields. This problem has heretofore limited the application of sensors heretofore in such environments.